All-optical networks, in which networks nodes are interconnected by single-mode optical fibers, often contain optical demultiplexers, optical switches, and optical multiplexers. Further, network nodes can be interconnected by counter-rotating rings, or in a mesh, ring, or bus topology. In the counter-rotating ring topology an increased network survivability to fiber failure can be achieved. The notion of counter-rotating ring is well-known in the art, and has been widely adopted in, for example, in the Fiber Data Distribution Interconnection (FDDI).
Multiple optical signals at different wavelengths can coexist in these fibers by using Wavelength Division Multiplexing (WDM) techniques. A typical network node of a WDM system includes the functions of demultiplexing a traffic signal, switching channels of the traffic signal to add and drop local channels, and multiplexing the switched channels for transmission. The network node typically includes a controlling unit for operations and maintenance (O/M) functions, and an optical demultiplexer, an optical switch, and an optical multiplexer.
Each optical signal within the network node passes through a number of elements within the optical demultiplexers, optical switch, and optical multiplexers. The optical channel transmission path through these elements is called a lightpath. A failure of the lightpath can occur if an element within the node fails to operate correctly, or fails completely. The element causing the failure can be difficult to locate.